One or more embodiments of the present invention relate to a power supply impedance optimizing apparatus which optimizes impedance of a power supply of a printed board on which a semiconductor device is mounted.
On a semiconductor chip or in a package of a semiconductor device, power supplies are sometimes separated from each other so as to prevent the noise from sneaking between different types of power supplies. However, separation of power supplies tends to degrade impedance characteristics and, as a result, impedance of each power supply is often increased. However, in order to reduce the noise, it is desirable for a power supply to have a lower impedance.
Also, depending on an operating condition, there are some other cases where it is not necessary to separate the power supplies and it is desirable to lower impedance of a power supply for a particular frequency band only.
Conventionally, as described in JP 2001-53231 A, JP 2008-76356 A, JP 2009-94133 A, and JP 2009-99718 A, a power supply noise detecting circuit or the like is provided inside a semiconductor chip; the capacity necessary for a power supply is adjusted based on a noise level of the power supply; and impedance of the power supply is optimized in accordance with the operating condition.
JP 2001-53231 A describes a monitor pad which is connected to a power supply terminal and a ground terminal of a silicon chip of an LSI. Power supply noise is measured directly by bringing a probe into contact with the monitor pad to determine whether the LSI needs a decoupling capacitor or not.
JP 2008-76356 A describes that a mutual induction inductor pair consisting of a first inductor connected between power supply voltages of an integrated circuit and a second inductor which is opposed to the first inductor and at its both ends connected to external output terminals of the integrated circuit is provided in the integrated circuit, and power supply noise of the integrated circuit is measured based on a voltage waveform output from the second inductor through the external output terminal.
JP 2009-94133 A describes a method in which power supply impedance is switched according to varied potential applied to a circuit which performs a predetermined process, so as to allow a resonance frequency of a semiconductor integrated circuit to be separated from an operation frequency of the circuit which performs the predetermined process.
JP 2009-99718 A describes a method in which a plurality of decoupling cells are mounted, and the number of switches to be ON is controlled to thereby regulate the number of decoupling cells to be connected to a power supply based on the noise level of the power supply that varies depending on operating conditions of an internal circuit.
However, it becomes useless to separate power supplies because power supply impedance is reduced in all frequency ranges, which may result in the increase in sneak noise.